1. Field of the Invention
The present invention relates generally to optical components, and particularly to tunable optical components.
2. Technical Background
Various optical components for use in optical communication systems, such as systems using wavelength division multiplexing (WDM), are known. Wavelength division multiplexing is commonly used to more efficiently utilize the scarce resource of bandwidth for high-speed data transmission in an optical fiber network. In a WDM system, each high-speed data channel transmits the information contained within the channel at a pre-allocated wavelength on a single optical waveguide, such as an optical fiber. By sharing the transmission medium of the common waveguide, multiple high speed data signals can be multiplexed for transmitting data to a distant location. -At the receiver end, channels of different wavelengths are separated by narrow-band filters and then detected, optically monitored or otherwise used for further processing. As WDM systems multiplex higher and higher densities of wavelength channels, filters of very narrow bandwidths become an increasing need. Such WDM systems require tunable or reliably fixed (or compensated) optical components to provide accurate band selection and spectral purity, along with other optical components which add to the WDM system complexity and reliability requirements.
Examples of tunable or fixed optical components include tunable Fabry-Perot (FP) filters and applications using the (FP) filtering principle to provide further wavelength selective components, such as circulators and one-port or multiport wavelength add/drop multiplexers or demultiplexers, modulators, variable optical attenuators, isolators, and switches. Such filters can be narrow-band or wide-band bandpass filters. When used as bandpass filters in the appropriate band, the FP filters provide for channel selection in wavelength-division-multiplexing.
Generally, Fabry-Perot (FP) filters, resonators, etalons, interferometers or other FP optical components having FP cavities are very simple devices and well-known in theory. Whether a FP component is called a filter, an etalon, an interferometer, or another name typically depends on personal preference and the particular application. One application for a tunable FP filter is in optical performance monitoring. By changing the cavity length of a high finesse Fabry-Perot filter, information about the optical signal-to-noise ratio and power level can be determined. As is known, the finesse (F) of the FP filter is a quality index that refers to the resolution of the filter. The monitoring of system performance is becoming more important due to system complexity and reliability requirements.
Quality monitoring of the signals circulating in optical fiber networks is typically performed by using high resolution spectral or interferometric analysis. For WDM mode transmission, the power and the signal-to-noise ratio of each channel is measured. An optical performance monitoring (OPM) module typically includes an electronic data processor along with the scanning optical filter. A scanning optical filter is just a fixed (or compensated) filter where the wavelengths are varied or scanned. In contrast, a tunable filter is a variable filter where the resonant wavelength of the filter can change by changing a parameter of the filter itself.
Basically, in a typical Fabry-Perot cavity, the cavity length determines important parameters; the free spectral range (FSR) and the resonant cavity frequency (or wavelength) of the resonator or filter. One common Fabry-Perot tunable filter approach utilizes piezoelectric or heat-sensitive actuators to change the cavity length by an appropriate amount to result in a tunable resonant frequency (or wavelength). These FP components can be based on cavities formed by the faces of two GRIN lens collimators or the cleaved facets or reflective faces of two optical fibers. The GRIN lens collimator is a short segment of a radially graded-index (GRIN) medium that can collimate light as the light propagates through the medium. In both cases of collimators or facets, care in alignment and other manufacturing factors must be taken to maintain a high cavity finesse F to assure a narrow bandwidth filter. For example, cavities with a finesse F value of xcx9c2000 correspond to a filter bandwidth of only 5 Ghz (or 40 pm).
The best spectral resolution is obtained with a Fabry-Perot interferometer (FPI) which is just a specific application of a FP filter in which the cleaved faces of opposing optical fibers are facing each other in an air gap of an air cavity. The air gap between the two faces can be made very small (a few xcexcm), so that the free spectral range (FSR) of the resonator or filter is wide. Each face is usually covered by a multilayer high reflectivity mirror. A lightwave, guided by the first fiber, exits partially into the air cavity, and is submitted to multiple reflections on both mirrors. Part of the energy is coupled into the second fiber, which is connected to an optical detector.
The optical alignment of the two fiber cores to ensure proper concentricity is a very critical manufacturing step. As the guided wave exits the first fiber, the guided wave diverges or diffracts while traversing the air gap. In the general classification of optical resonators, the plane-plane air cavity is xe2x80x9cunstablexe2x80x9d and can have a low finesse F value. In a fiber based FP cavity, the expansion of the optical mode across the cavity degrades the cavity finesse F. One solution to reduce the optical loss of such a resonator, consists in introducing in the resonator a piece of fiber, whose thickness is very precisely determined. In a commercially available device, a short section of fiber (fiber stub or wafer) is used inside the cavity to maintain the optical mode size and hence the cavity finesse. However, this method is time consuming, and leads to high manufacturing cost.
Various forms of FP components are known, including advanced technological structures based on liquid crystals, microelectro-mechanical systems (MEMS) and polymer films deposited on a non-expanding substrate, that may still have to be carefully aligned with the transmissive medium. However, current tunable optical filters that are available and that have gone through extensive reliability testing are much simpler mechanically, as with a cavity created in a fiber ferrule assembly, but can be as expensive as $10,000 a piece.
Accordingly, a need exists for a high finesse FP optical component that can be inexpensively and easily created, exhibit a practical manufacturable form, and minimize losses from misalignments.
One aspect of the present invention is the multiple uses made available of an optical component that is based on a fiber Fabry-Perot resonator. The optical component includes a substrate having a variable length for supporting the Fabry-Perot resonator by varying the length of the substrate in response to a stimulus. A plurality of fiber retainers are disposed on the substrate for mounting and aligning the fiber Fabry-Perot resonator. To fix the position of the fiber Fabry-Perot resonator relative to the substrate and to define the variable length, a pair of binders are disposed on the substrate.
In another aspect, the present invention uses a heat-sensitive substrate such as silicon, silica or polymer.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.